Designers are increasingly using distributed power supply architectures for large electronic equipment. With this type of architecture, electrical power is bussed throughout the equipment at a relatively high dc voltage, such as 48 volts. dc/dc converters mounted near the load (and often on the same printed circuit board as the load) then step this high voltage down to the low voltage required by the load (e.g. 3.3V), typically through an isolating transformer.
These point-of-load dc/dc converters typically have a low height (e.g. 0.5xe2x80x3) so that the designer can place adjacent load boards close together in a card rack. The plan-view size of the converter must also be as small as possible to leave more room on the load board for the load circuitry. Several standard sizes of converters exist, such as the xe2x80x9cFull Brickxe2x80x9d (2.4xe2x80x3xc3x974.6xe2x80x3), the xe2x80x9cHalf-Brickxe2x80x9d (2.4xe2x80x3xc3x972.3xe2x80x3), and the xe2x80x9cQuarter-Brickxe2x80x9d (2.4xe2x80x3xc3x971.45xe2x80x3). Other standard and non-standard sizes exist, as well. In general, the larger a dc/dc converter, the more power it can handle.
Typically, dc/dc converters have a flat bottom surface formed by either a housing or potting material. Terminal pins extend from this surface so that the dc/dc converters can be xe2x80x9cthrough-hole mountedxe2x80x9d on a printed circuit board (the xe2x80x9cPCBxe2x80x9d). When the converter""s xe2x80x9cthrough-hole pinsxe2x80x9d are inserted into the PCB""s holes, the bottom surface of the converter makes contact with the PCB to ensure its proper positioning in the z-axis direction.
Recently, xe2x80x9copen framexe2x80x9d converters have been developed without a housing or potting. To achieve proper z-axis positioning, these converters use plastic or metal xe2x80x9cstandoffsxe2x80x9d that keep the PCB and the converter""s substrate separated by a specified distance. Because these standoffs either abut or are attached to the converter""s substrate, they take up space on the substrate that could otherwise be used for electronic components. They also partially or totally block the cooling air from flowing under the open frame converter. Finally, the standoff represents an additional cost for the part and for its attachment to the converter.
Most electronic equipment manufactured today uses Surface Mount Technology (SMT) to attach their components to both the top and bottom surfaces of a PCB. In this process, solder paste is first screen-printed onto the PCB in the locations of the component pads. The components are then placed onto the solder paste. Finally, the PCB is passed through a reflow oven in which the solder paste melts and then solidifies during the cool-down stage.
In comparison, dc/dc converters, with their through-hole pins, are attached to the PCB by either manual soldering or by an automated production process called xe2x80x9cwave solderingxe2x80x9d. With this latter process, the PCB is first preheated and then passed over a molten pool of solder. The solder comes in contact with the bottom of the PCB, and it wicks into the through-holes and solidifies after the PCB leaves the pool of solder.
A typical manufacturing process that requires both SMT and wave soldering would first attach the SMT parts on the PCB, then insert the through-hole components, and finally pass the PCB through the wave soldering machine. This process requires that the SMT components mounted on the bottom side of the PCB pass through a molten pool of solder.
As the distance between the leads on SMT packages gets smaller, it becomes more difficult to pass these packages through a wave solder process and not have solder bridges form between adjacent leads. Furthermore, the heating associated with the wave soldering process compromises the quality of the SMT components and their attachments to the PCB. Manufacturers of electronic equipment are therefore interested in avoiding the use of wave soldering altogether. Often, the dc/dc converter is the only component on their boards that requires wave soldering.
In response to this desire, several power supply manufacturers have created dc/dc converters designed to be surface mounted to a PCB. Instead of a few, large diameter through-hole pins, some of these converters have many smaller leads designed for surface mounting. In general, these surface mount pins make a dc/dc converter""s overall footprint larger than it might otherwise be since the pins typically extend beyond the converter""s original footprint. Alternatively, at least one manufacturer has introduced a product that uses a surface mountable ball-grid. In this product, each through-hole pin of a standard converter is replaced with a conductive ball of sufficient diameter to permit the converter to be attached to the PCB with SMT techniques.
One important problem with all of these approaches for making a surface mountable dc/dc converter is the relative weakness of a surface mount joint compared to a through-hole pin. This problem is particularly important since dc/dc converters have a higher mass than most components, and the mounting joints are therefore more susceptible to shock and vibration stresses.
Another problem with a surface mountable dc/dc converter is that the converter""s pins make electrical contact with only the outer conductive layer in the PCB. Normally, the PCB""s power and ground planes use inner conductive layers. With a surface mount connection, additional vias (that take up space and add resistance) are therefore required to connect the outer conductive layer to the inner ones.
In comparison, a through-hole mounting is much stronger mechanically. It also provides direct electrical attachment of the pin to the inner conductor layers of the PCB.
What is needed is a way to solder a through-hole mounted pin with a reflow solder process, instead of using manual or wave soldering.
To address the problems mentioned above, a new through-hole terminal pin is used for mounting dc/dc converters or other circuit modules. In one embodiment, this pin is similar to a standard through-hole pin, but it has a circular flange near its bottom end. The diameter of the flange is greater than the diameter of the PCB hole through which the lower portion of the pin is inserted. The bottom of the flange therefore rests against the PCB""s surface. It is located a specified distance from the dc/dc converter""s substrate so that it provides the function of a stand-off, but without taking up space on the substrate or requiring a separate part. In addition, its interference with the cooling airflow underneath the dc/dc converter is minimal.
In another embodiment, the through-hole pin has a flange near or at the top end of the pin where it makes contact with the dc/dc converter""s substrate. The top of this flange rests against the bottom of the substrate. This arrangement improves the mechanical connection of the pin to the dc/dc converter""s substrate, and it provides one way to ensure the proper z-axis placement of the pin relative to the substrate.
In a third embodiment, the through-hole pin has one continuous, larger diameter portion that performs the function of separate flanges on either end.
In a fourth embodiment, the end of the pin has a cross-sectional shape that is pointed along its periphery. This pointed shape facilitates press fitting, or swaging, the pin into a hole of either the substrate, the PCB, or both. The press fit holds the pin in place for later soldering in a hand, wave, or reflow process, and it improves the mechanical strength between the pin and the substrate or PCB.
In addition, a process has been invented to permit this new through-hole pin to be soldered to the PCB with a reflow process, instead of using manual or wave soldering. In one embodiment, this process works as follows.
First, the pad around the PCB""s through-hole is designed to be commensurate in size and shape with the flange of the converter pin.
Second, solder paste is screen-printed onto the PCB in the locations of the pads for both the SMT components and the dc/dc converter pins.
Third, both the SMT components and the dc/dc converter are placed on the PCB. The dc/dc converter, since it is relatively large and heavy, might be placed manually or by a special machine, although it could be placed by the same machine as the other SMT components. At this point, the bottoms of the flanges sit on top of solder paste, while the lower parts of the through-hole pins are inserted into their PCB holes.
Finally, the PCB is passed through a reflow oven in which the solder paste first melts and then solidifies. During this step, the solder paste between each pin flange and the PCB wicks down into the corresponding PCB hole. The final solder joint between the pin and the PCB will therefore exist both underneath the flange and inside the PCB hole. With a properly designed pad and screening process, there will also be a fillet of solder around the outer edge of the flange to provide additional mechanical stress relief. The result is a very strong mechanical connection between the pin and the PCB, as well as a low resistance electrical connection between the pin and both the inner and outer conductive layers of the PCB.
The flange facilitates this special soldering process. It provides a region in which the solder paste directly contacts both the pin and the PCB. As the solder melts, it readily wicks along the surface of the flange and down the pin such that it fills the gap between the pin and the PCB hole""s via metalization.
In another embodiment of the reflow soldering process, the bottom end of the through-hole pin is given a cross-sectional shape that is pointed. When the pin is press fit into the PCB, the points of the pin hold the pin, and therefore the converter, in place. Solder is then applied to the bottom side of the PCB in the region of the hole and its pad. The PCB is then passed through a reflow oven in which the solder paste melts, flows into the gaps between the pin and the hole, and then solidifies.
In this alternative reflow soldering process the end of the inserted pin should not extend beyond the bottom of the PCB. Otherwise, it might interfere with the solder application step. In fact, it is useful for the end of the inserted pin not to reach the bottom side of the PCB (i.e., for the end to be inside the PCB). Such an alignment gives a small xe2x80x9cwellxe2x80x9d in the hole area, which increases the amount of solder that can be applied in this area. A flange near the bottom end of the pin facilitates the correct insertion depth of the pin into the PCB hole, although there are other well-known means for controlling this depth.
This alternative reflow soldering process can also be used to attach the pin to the dc/dc converter""s substrate during the construction of the converter.
Thus, in accordance with one aspect of the invention, a dc/dc converter comprises a converter substrate having circuitry thereon. At least one rigid terminal pin directly attaches to the converter substrate and is electrically connected to the circuitry. The terminal pin includes a flange having a shoulder to abut a printed circuit board into which the pin is inserted and to which electrical connection is made. The shoulder may abut the printed circuit board by making direct contact thereto, or through one or more layers of material, such as solder. The shoulder is spaced from the converter substrate to accommodate spacing of the converter substrate from the printed circuit board. Plural pins may together provide the spacing between the converter substrate and the printed circuit board or one or more pins may operate with more conventional standoff mechanisms.
To allow for a subsequent soldering process to, for example, solder the terminal pin to the printed circuit board, the components, materials and solder connections of the converter may be such that they are not adversely affected by a 210xc2x0 soldering process. In particular, the solder used on the converter substrate has a melting temperature greater than 210xc2x0 C.
The terminal pin may have a second shoulder which abuts the converter substrate. For example, the second shoulder may be on a second flange with the pin extending from the second flange into the converter substrate. The second flange may be spaced from the first flange. Alternatively, a single flange may extend along a length of a terminal pin to abut both the printed circuit board and the converter substrate. In one embodiment, the flange has a uniform diameter.
The terminal pin may be swage fit into the converter substrate. To that end, the pin may have a pointed cross-section shape. The portion of the terminal pin extending into the converter substrate may also be soldered to the converter substrate such as by a reflow soldering process.
The invention is particularly suited to a converter substrate having circuitry thereon in an open frame construction without a baseplate where the converter substrate is positioned parallel to the printed circuit board.
In accordance with another aspect of the invention, a dc/dc power converter is mounted to a printed circuit board by soldering the converter to the printed circuit board with the terminal pin extending through a circuit board hole and the shoulder of the terminal pin abutting the circuit board to accommodate spacing of the dc/dc converter from the circuit board. Preferably, the solder is applied to the circuit board or shoulder, and the shoulder is thereafter positioned to abut the printed circuit board through the solder. The solder may be applied as a solder paste about the circuit board hole, and the hole may be left substantially free of solder paste when the paste is applied. The assembly may thereafter be subjected to a solder reflow process.
The solder may flow to form a fillet. For example, the solder may flow radially to form a fillet about the flange. The solder may also flow through the hole in the printed circuit board to form a fillet about a portion of the terminal pin exposed beyond the circuit board hole.
Solder may be applied to the holes from an opposite board side of the printed circuit board after insertion of the terminal pins into the holes. Specifically, the solder may be applied from a molten pool of solder positioned below the printed circuit board.
In accordance with another aspect of the invention, solder is preapplied on the shoulder of the flange. For example, the solder on the flange may be in a paste, may be a preform, or may be coated on the shoulder of the flange.